Capturing a quality image when the scene to be captured includes low light or fast motion is challenging. If a long exposure is used to increase the amount of light that is collected, thereby reducing the noise in the image, any motion present from the camera or the movement of objects within the scene results in motion blur artifacts in the image. Alternately, if a short exposure is used to reduce motion blur, noise will be present in the image due to the low signal at the image sensor. One solution to this photographic challenge is to capture multiple short exposure images which each have reduced motion blur due to their short exposure times. The multiple short exposure images are then combined in a way that compensates for the motion that occurs between the images to produce an improved image which effectively has an exposure time that is equal to the combined exposure time of the multiple short exposure images and an equally higher combined signal with reduced noise due to averaging between the multiple images.
An overlapped readout technique for an image sensor that produces an image with reduced motion blur is described in co-pending United States Patent Application 2009/0021612. Charge from a portion of the photodetectors is transferred into the vertical charge-coupled device (VCCD) shift registers followed by readout of a component image. This technique may extend the time between the overlapped images because additional time may be required to readout the component image from the image sensor. As a result, it may be difficult to capture a series of images with short exposure times in rapid succession to provide a series of images that accurately illustrate the motion that is occurring in the scene.
Another solution to this photographic challenge is disclosed in United States Patent Application 2006/0017837. A series of images are captured, some with short exposures and some with long exposures. A combined image is then produced that has reduced motion blur from the short exposure images and reduced noise from the long exposure images. However, the resulting large difference in length of exposure times between the short exposure image and the long exposure may make it difficult to combine the two images. Also since the two exposures are captured at different times it may be difficult to align the two images when motion is present, particularly if there is local motion present produced by objects moving within the scene.
A method for capturing multiple images in rapid succession using a CCD sensor is disclosed in United States Patent Application 2007/0002165. A first image is captured and the charge transferred into the VCCD using only the rows associated with a first field. The photodetectors are then reset and a second image is captured and transferred into the VCCD using only the rows associated with the second field. The photodetectors are reset again before a third image is captured. The VCCD is then readout which reads the first and second images into the horizontal charge coupled device (HCCD) and converts the charge data for the two images into digital data. The third image is then transferred to the VCCDs, readout, and converted to digital data. While this method reduces the time between captures since the time between captures is not increased by having to wait for the image sensor readout, the photodetectors are reset for each image, so that the images are all captured at different times. The alignment of the multiple images may be challenging in some situations.
A dual capture mode of operation is described in a paper entitled “A CCD Image Sensor and Imaging System with Scene Adaptability for DSC Applications” (Int Sym Tech for Digital Photo Fulfillment, Soc, Imaging Sci & Tech, February 2009) by M. Kobayashi, S. Tanaka, K. Oda, K. Ikeda, K. Hayashi, and T. Nishimura. A CCD image sensor is used to capture one image with a long exposure time along with a second image having a short exposure time. The two images are captured with different rows of pixels in the same image sensor. A “sweep-out process” is used to reset the pixels that are used to provide the second short exposure image. As such, each pixel is used in only one of the two images and the method is limited to capture of two images in a set.